The proteins immunochemically related to P-450 HFLa, a major form of cytochrome P-450 in human fetal livers, are present in liver microsomes from various animal species.

Proteins immunochemically reactive with anti-P-450 HFLa IgG were detectable in liver microsomes from all of the animals examined, although considerable variations of the amounts were observed among the animal species. The smallest amount was found in liver microsomes from female rats and the largest amount in monkey liver microsomes. Sex difference in the amounts was observed in rat liver microsomes but not in dog liver microsomes. No strain difference was observed among ddY, ICR and BALB/C mice. The activities of testosterone 6 beta-hydroxylases in liver microsomes from rats, dogs and monkeys were highly sensitive to inhibition by anti-P-450 HFLa IgG, but those in microsomes from guinea pigs and rabbits were less sensitive to inhibition by the antibodies.